Plasma torches are an important component of inductively coupled plasma devices which are used in elemental analysis. Inductively coupled Plasma (ICP) Torches commonly include three concentric tubes including an inner sample tube for delivering an aerosol sample to the plasma, the middle tube (frequently flared) for delivering the plasma gas, and the outer tube for delivering a coolant gas.
The three tubes are typically in concentric alignment over a length of 5-20 centimetres, and the required tolerance is very fine. For example the distance between the flared end-section of the plasma tube and the surrounding coolant tube is of the order of 0.4 mm, and the positioning of the plasma tube must be precise preferably to within 50 micrometres. As a result ICP torches have conventionally been formed using three tubes fused at one end to fix the tolerances required for consistent operation.
A torch in which the tubes are fused is difficult to clean or repair, for example when the sample tube is contaminated or when the coolant tube undergoes melt-down.
Attempts have been made to produce demountable torches by using a base structure in which the tubes are removably mounted. The demountable base assembly must however anchor all the tubes at a distance of at least 5 cm from the point of greatest sensitivity (ie the place where the flared end of the plasma tube is 0.4 mm from the coolant tube. Frequently the insertion of a new tube in a demountable base assembly is labour intensive and involves such tasks as performing trial rotations of one or more tubes to avoid pinch points in the gas flow.
U.S. Pat. No. 4,833,294 (A. Montaser et al. filed Feb. 12, 1998) describes a plasma torch including a base member, a plasma tube and a threaded insert member within the plasma tube for directing the plasma gas in a tangential flow pattern, and for sample introduction. This design comprises only one tubular member inside another, however high precision alignment is still necessary, particularly since the two tubular members are closer together than the concentric tubes in a standard 3-tube ICP torch. Furthermore the torch design in U.S. Pat. No. 4,833,294 has been developed to support a helium plasma at atmospheric pressure, and so the design is not useful for many ICP units which use argon plasmas.
The problem of reliably and reproducibly securing a highly concentric configuration of tubes (generally 3 tubes) in a demountable apparatus has previously been addressed. A review of prior art up to 1991 has been provided in U.S. Pat. No. 5,186,621 (filed Jul. 22, 1991). A discussion is provided of type I assemblies which rely on three compressed O-rings to define the concentric configuration (together with guide elements), and to inhibit gas leakage—a first compressed O-ring to provide a seal between the outer portion of the largest-diameter tube and the upper tube guide, a second compressed O-ring to provide a seal between the upper and lower tube guides, and a third compressed O-ring to provide a seal between the injector tube and the tube port in the lower tube guide. The O-rings are compressed by placing a shield over the tube/tube guide combination, affixing the shield to a base having a bore there-through, and screwing a compression plug forcibly into the base. The disadvantages of the above-described compressed O-ring system include:                The large number of required parts        The need to apply high pressure with the compression plug, which may cause breakage of a quartz tube or an injector tube        The system is prone to O-ring failure, particularly failure arising from heat and UV radiation encountered when the torch is operational. O-ring failure leads to gas leakage and possibly arc formation or non-uniformity in sample flow        The system often provides poor reproducibility between successive results.        
In the same patent a discussion is provided of type II assemblies—these have a single blown quartz element that combines the coolant and plasma tubes, and a separate segmented attachment capable of accommodating injector tubes. Type II assemblies suffer from many of the shortcomings of type I assemblies. It is stated that leaks generally occur after one or two weeks of use due to heat and UV degradation of the injector tube O-ring. U.S. Pat. No. 5,186,621 goes on to describe a quartz element containing two outer tubes with gas entry ports (all fused together) and a superior double O-ring system for securing the sample tube. A compression plug is used to apply pressure to all sealing O-rings. However clearly the fused quartz element is a complex manufacture, and breakage or melting of any part of this element will entail its entire replacement.
U.S. Pat. No. 4,266,113 (filed Jul. 2, 1979) describes the use of spacer rings slip-fitted into the coolant and plasma passages of a plasma torch to provide support for the concentric arrangement and to also permit dismounting. Each ring is slotted to either direct coolant gas in a spiral flow pattern to improve its heat flow properties or to enhance the laminar character of the plasma gas flow. In a system of this kind, it is difficult to maintain the necessary tolerances for continued optimal function of the ICP torch particularly after multiple replacements of components.
U.S. Pat. No. 4,739,147 (filed Jan. 30, 1987) describes a plasma torch wherein the three tubes that are generally used in demountable plasma torches are joined together with taper joints. The axes of the taper joints are centrally aligned with the central long axis of the tubes so that the tubes are pre-aligned and concentric in the assembled torch. The assembly consists of only 3 parts. Such an arrangement must be fabricated by a glass-blower by welding together quartz components. It is very difficult to reproducibly provide an accurate location of all the tubes (including all future replacements) in order to achieve optimal operation of a plasma torch over an extended period.
The above discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of this application.
We have prepared demountable plasma torches comprising a base which is adapted to receive three tubular elements held in place by sliding friction. The sleeve which provides the precision engineered inner surface profile that can receive 3 tubes by sliding friction is expensive to make (often half or more of the cost of the complete torch is expended in the base component). Furthermore the fully assembled torch is somewhat bulky, and a special mounting bracket is required to connect the torch with standard argon ICP instruments.
There is a need to provide an ICP torch that matches the performance of the fused tube products but which can be readily disassembled for cleaning and replacement of components.